通用委托方差编译错误

Question

I'm using the MSDN examples for variance in delegate . But the following code is giving me a compilation error. Based on my understanding, it should accept the First as an argument. What am I doing wrong? The code sample.

SampleGenericDelegate<Second, First> secdel = AFirstRSecond;
secdel(new First()); //compilation error here.

public class First { }
public class Second : First { }
public delegate R SampleGenericDelegate<A, R>(A a);
public static Second AFirstRSecond(First first)
{ 
    return new Second(); 
}    

Errors

Delegate 'ConsoleApplication1.SampleGenericDelegate<ConsoleApplication1.Second,
ConsoleApplication1.First>' has some invalid arguments

Argument 1: cannot convert from 'ConsoleApplication1.First' to    
'ConsoleApplication1.Second'    

Answer 1

secdel has an argument type of Second , so you need to pass an instance of that:

SampleGenericDelegate<Second, First> secdel = AFirstRSecond;
secdel(new Second());

Answer 2

The first type in the generic definition is the type used for defining the parameter of the delegate, not the return type. Change your code as such and you should be good.

SampleGenericDelegate<First, Second> secdel = AFirstRSecond;
secdel(new First()); //compilation error here.

public class First { }
public class Second : First { }
public delegate R SampleGenericDelegate<A, R>(A a);
public static Second AFirstRSecond(First first)
{ 
    return new Second(); 
}